//
// timer.v
//
// Gameboy for the MIST board https://github.com/mist-devel
// 
// Copyright (c) 2015 Till Harbaum <till@harbaum.org> 
// 
// This source file is free software: you can redistribute it and/or modify 
// it under the terms of the GNU General Public License as published 
// by the Free Software Foundation, either version 3 of the License, or 
// (at your option) any later version. 
// 
// This source file is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of 
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the 
// GNU General Public License for more details.
// 
// You should have received a copy of the GNU General Public License 
// along with this program.  If not, see <http://www.gnu.org/licenses/>. 
//

module timer (
	input  reset,
	input  clk_sys,
	input  ce,    // 4 Mhz cpu clock
	output reg irq,
	
	// cpu register interface
	input  cpu_sel,
	input [1:0] cpu_addr,
	input  cpu_wr,
	input [7:0] cpu_di,
	output [7:0] cpu_do,
	
	// savestates              
	input  [63:0] SaveStateBus_Din, 
	input  [9:0]  SaveStateBus_Adr, 
	input         SaveStateBus_wren,
	input         SaveStateBus_rst, 
	output [63:0] SaveStateBus_Dout
);

// savestates
wire [46:0] SS_Timer;
wire [46:0] SS_Timer_BACK;

eReg_SavestateV #(0, 6, 46, 0, 64'h0000000000000008) iREG_SAVESTATE_Timer (clk_sys, SaveStateBus_Din, SaveStateBus_Adr, SaveStateBus_wren, SaveStateBus_rst, SaveStateBus_Dout, SS_Timer_BACK, SS_Timer);  

// input: 4Mhz
// clk_div[0] = 2Mhz
// clk_div[1] = 1Mhz
// clk_div[2] = 524khz
// clk_div[3] = 262khz
// clk_div[4] = 131khz
// clk_div[5] = 65khz
// clk_div[6] = 32khz
// clk_div[7] = 16khz
// clk_div[8] = 8khz
// clk_div[9] = 4khz

wire resetdiv = cpu_sel && cpu_wr && (cpu_addr == 2'b00); //resetdiv also resets internal counter

reg [9:0] clk_div;
reg clk_div_1_9;
reg clk_div_1_3;
reg clk_div_1_5;
reg clk_div_1_7;
always @(posedge clk_sys)
	if (reset)
		clk_div <= SS_Timer[9:0]; // 10'd8;
	else if(resetdiv)
		clk_div <= 10'd2;
	else if (ce)
		clk_div <= clk_div + 10'd1;

reg [7:0] div;
reg [7:0] tma;
reg [7:0] tima;
reg [2:0] tac;
reg tima_overflow;
reg tima_overflow_1;
reg tima_overflow_2;
reg tima_overflow_3;
reg tima_overflow_4;

assign SS_Timer_BACK[ 9: 0] = clk_div;
assign SS_Timer_BACK[17:10] = tima;
assign SS_Timer_BACK[25:18] = tma;
assign SS_Timer_BACK[28:26] = tac;
assign SS_Timer_BACK[29]    = irq;
assign SS_Timer_BACK[37:30] = div;
assign SS_Timer_BACK[38]    = clk_div_1_9;
assign SS_Timer_BACK[39]    = clk_div_1_3;
assign SS_Timer_BACK[40]    = clk_div_1_5;
assign SS_Timer_BACK[41]    = clk_div_1_7;
assign SS_Timer_BACK[42]    = tima_overflow;
assign SS_Timer_BACK[43]    = tima_overflow_1;
assign SS_Timer_BACK[44]    = tima_overflow_2;
assign SS_Timer_BACK[45]    = tima_overflow_3;
assign SS_Timer_BACK[46]    = tima_overflow_4;

always @(posedge clk_sys) begin
	if(reset) begin
		tima            <= SS_Timer[17:10]; // 0
		tma             <= SS_Timer[25:18]; // 0
		tac             <= SS_Timer[28:26]; // 0
		irq             <= SS_Timer[29];    // 0
		div             <= SS_Timer[37:30]; // 0
		clk_div_1_9     <= SS_Timer[38];    // 0
		clk_div_1_3     <= SS_Timer[39];    // 0
		clk_div_1_5     <= SS_Timer[40];    // 0
		clk_div_1_7     <= SS_Timer[41];    // 0
		tima_overflow   <= SS_Timer[42];    // 0
		tima_overflow_1 <= SS_Timer[43];    // 0
		tima_overflow_2 <= SS_Timer[44];    // 0
		tima_overflow_3 <= SS_Timer[45];    // 0
		tima_overflow_4 <= SS_Timer[46];    // 0
	end else if (ce) begin
		irq           <= 1'b0;

		tima_overflow   <= 1'b0;
		tima_overflow_1 <= tima_overflow;
		tima_overflow_2 <= tima_overflow_1;
		tima_overflow_3 <= tima_overflow_2;
		tima_overflow_4 <= tima_overflow_3;

		if(clk_div[7:0] == 0)   // 16kHz
			div <= div + 8'd1;

		clk_div_1_9 <= clk_div[9];
		clk_div_1_3 <= clk_div[3];
		clk_div_1_5 <= clk_div[5];
		clk_div_1_7 <= clk_div[7];

		// timer enabled?
		if(tac[2]) begin
			// timer frequency, count up when uppermost clk_div bit switches from 1 to 0
			if(((tac[1:0] == 2'b00) && (!clk_div[9] && clk_div_1_9)) ||     // 4 khz
				((tac[1:0] == 2'b01) && (!clk_div[3] && clk_div_1_3)) ||     // 262 khz
				((tac[1:0] == 2'b10) && (!clk_div[5] && clk_div_1_5)) ||     // 65 khz
				((tac[1:0] == 2'b11) && (!clk_div[7] && clk_div_1_7))) begin // 16 khz

				tima <= tima + 8'd1;
				if(tima == 8'hff) begin
					tima_overflow <= 1'b1;
				end
			end
		end
		
		if (tima_overflow_3) begin // 1 cycle = 4 clock cycles after timer overflows
			irq <= 1'b1;    // irq 
			tima <= tma;    // reload timer
		end
		
		if (tima_overflow_4) begin // delay 4 is required because cpu write takes place 1 clock cycle later(too late?)
			if(cpu_sel && cpu_wr && cpu_addr == 2'b10) begin // writing tma when loading tima has instant effect
				tima <= cpu_di;
			end
		end
		
		if(cpu_sel && cpu_wr) begin
			case(cpu_addr)
				2'b00:  div <= 8'h00;    // writing clears counter
				2'b01:  begin
					if (!tima_overflow_4) begin 
						tima <= cpu_di;
					end
					tima_overflow_1 <= 1'b0; // prevent irq and loading of tima with write to tima at the same time
				end
				2'b10:  tma <= cpu_di;
				2'b11:  tac <= cpu_di[2:0];
			endcase
		end
	end
end

assign cpu_do = 
	(cpu_addr == 2'b00)?div:
	(cpu_addr == 2'b01)?tima:
	(cpu_addr == 2'b10)?tma:
	{5'b11111, tac};
	
endmodule